SH (orig.): Concentrated on boats and Pacific submarine war in great detail. Interesting, challenging and entertaining. UBI wasn't involved.
SHII: UBI gives a try. Stock didn't have Dynamic campaign, but boats still had couple interesting features (diesel/electric management) to be lost in the future.
SHIII: Someone - probably in this forum - says two magic words "Crew" and "Dynamic campaign". UBI listens and both are in. Boats are clearly degenerating.
SHIV: Goes back to Pac. Creates mixed feelings among users. On boats "working periscopes" are probably the biggest leap ahead. U-boat addon offers fantasy campaign and units to player, and "special abilities" to crew.
SHV: UBI's probably most secret submarine simulation project up to date, planned to be jealously guarded by DRM. Going back to Atlantic, Full Dynamic Campaign is forgotten again, and only one type of three main U-boat classes is left. Crew - with ( their very) "special abilities" - is probably coming to be even more important than before, so don't forget SOUP!
-RC-